1. Field of the Invention
The present invention relates to a solid state imaging device, a method of manufacturing the solid state imaging device, and a digital camera that utilizes the solid state imaging device, and particularly relates to a solid state imaging device that includes an amplification element for each pixel or for each group of plural pixels.
2. Related Background Art
Currently, a photodiode is generally used as a light receiving element that converts an optical signal into an electric signal through light absorption. An active sensor (APS: Active Pixel Sensor), which amplifies a charge subjected to photoelectric conversion with the photodiode by means of an amplification element provided per pixel (or plural pixels) to generate a detection signal, is commercially available as a solid state imaging device provided in, for example, a digital camera.
In such a sensor, the charge generated in the photodiode is transferred by a transfer transistor. At this point, when a lattice defect exists on an interface of an oxide film under a gate of the transfer transistor, the charge generated in the photodiode is trapped by the defect, causing an incomplete transfer of the charge. Thus, in a state in which such a defect exists, and particularly in the case where a small amount of light is to be detected, it is hard to detect the corresponding signal. Therefore, the above-described defect causes a lowering of sensitivity, and becomes a cause for generating a dark current or noise.
In order to suppress various kinds of influences caused by defects on the interface of the oxide film, an interface level density needs to be reduced by reducing a defect density. Further, a defect due to a dangling bond on the interface of the oxide film is dominant over various kinds of defects as a defect that generates a carrier trap. Therefore, terminating the dangling bond with hydrogen is effective for reducing the influences caused by defects. Terminating the dangling bond with hydrogen is realized by forming a film containing hydrogen in the vicinity of a light receiving element and subjecting the film to heat treatment. In general, a plasma silicon nitride film as a surface protection film, and a side-wall-shaped plasma silicon nitride film disposed in the vicinity of the light receiving element each function as a hydrogen containing film for a hydrogen supply source (JP 06-112453 A, JP 2003-282856 A (corresponding to US 2003096438 A)).
Moreover, a method has been proposed in which hydrogen is effectively supplied to a semiconductor substrate while a sensitivity of a light receiving element is maintained (JP 2000-012822 A (corresponding to U.S. Pat. No. 6,166,405)).
Description will be made of such a conventional method with reference to FIG. 10. FIG. 10 is a sectional view of a light receiving element portion in a prior art. In FIG. 10, an n-type light receiving element 806 and an n-type semiconductor region 807 functioning as a detection portion are formed in a p-type well 804 on a surface of a semiconductor substrate made of silicon. Further, on the semiconductor substrate, a transfer electrode 809 made of polycrystalline silicon is formed on a region between the light receiving element 806 and the n-type semiconductor region 807 through an oxide film 803 formed through thermal oxidation. A photocharge generated in the light receiving element 806 is transferred to the n-type semiconductor region 807 through the transfer electrode 809. Further, an antireflection film 810, which is composed of a silicon nitride film, is formed above the light receiving element 806. The antireflection film 810 is formed over the light receiving element 806 and the transfer electrode 809. In the prior art, the antireflection film 810 is formed to cover the light receiving element 806 and not to cover a part of the transfer electrode 809. As a result, the inhibition of hydrogen supply to an interface of the oxide film under the transfer electrode 809 is reduced while sensitivity of the light receiving element 806 is maintained.
Further, a structure in which a plasma nitride silicon film is used as a lens forming film is disclosed in each of JP 10-229180 A and JP 06-204443 A. Moreover, it is disclosed in JP 06-204443 A that, in the case where an insulation film configured to be a microlens is composed of a film containing hydrogen such as an SiN film deposited by ECR-CVD in a CCD type solid state imaging device, hydrogenation of a silicon substrate is performed, which effectively suppresses a dark current.
In general, as the amount of hydrogen supply increases from, for example, a plasma silicon nitride film formed as a surface protection film or the like, an amount of hydrogen termination of a dangling bond can be increased. Therefore, it is effective in reducing the influences due to defects that the film that works as a hydrogen supply source be thickened.
However, when the thickness of the surface protection film increases, an internal stress in the film increases accordingly. As a result, a damaged portion such as a crack tends to occur. Further, it is considered that an expansion or tensile stress is applied to the film depending on the temperature of heat treatment after the formation of the surface protection film that works as the hydrogen supply source, as a result of which the crack is apt to occur. Therefore, a slight increase in thickness may cause problems in stability and reliability of an element. Further, JP 06-204443 A presents a problem in that, in the CCD type solid state imaging device, the microlens is formed in a self-aligning manner in an opening portion of a light shielding metal, so that hydrogen is not sufficiently supplied to a portion except for the region of a light receiving element, for example, a portion under a gate of a MOS transistor. Therefore, it has been difficult for the structure of the CCD type solid state imaging device to be simply applied to a MOS type solid state imaging device. As regards, in particular, the MOS type solid state imaging device in which a MOS transistor is used as a switching element, a structure including plural wiring layers is generally adopted in most cases. There has been a case where, when a film, which is capable of supplying hydrogen to a semiconductor substrate, is provided on the wiring layers, the distance between the hydrogen supply film and the semiconductor substrate is long, and thus, suitable hydrogenation treatment is not performed.